Total joint replacement is an orthopedic technique in which the diseased ends of bone at a joint are removed and replaced by a combination of metal and plastic prostheses. Joint replacement surgery is done to relieve pain and retain motion of the joint. Total joint replacement is very successful in the larger joints of the lower extremity, the hip and knee. Recently, new techniques and prosthetic designs have made this procedure more common as a treatment option for ankle arthritis.
During total joint replacement surgery precise cuts are made in the ends of the bones by the joint to remove the diseased joint surfaces. A metal appliance, or prosthesis, is then applied on the end of each bone, with a plastic spacer between them. Each joint has its own unique anatomy and functional characteristics. Different prosthetic designs require bone cuts with unique geometry. For the ultimate joint function to be optimal, bone cuts must be accurate so that the prostheses are placed in the proper alignment and orientation.
Currently, there are several designs of total ankle replacement prostheses. One type of total ankle replacement prosthesis is sold by Wright Medical Technology, Inc. (5677 Airline Road, Arlington, Tenn. 38002, USA) under the trademark INBONE Total Ankle System. The INBONE Total Ankle System includes a large intramedullary stem on the tibial side. Although the existing technique used to define the bone cuts for this prosthesis can be accurate, it is very complex thereby rendering it problematic to work with. Specifically, the existing technique or defined algorithm that is used during surgery to determine the alignment and position of cuts in the bone for the correct insertion of this prosthesis is as follows.
First, the anterior ankle is exposed through a longitudinal incision. The leg, ankle and foot are then placed into a leg-holding frame, with the ankle in neutral dorsiflexion and plantarflexion.
The leg is manipulated under fluoroscopy to show a true mortise view. The mortise view is a standard radiographic description of an internally rotated view of the ankle that looks down the axis of the body of the talus between the medial and lateral malleolei. The x-ray beam is perpendicular to the intermalleolar axis. In this situation the intermalleolar axis is parallel to the top of the operating table. Once this position is established, it is maintained by fixing the foot into the leg holder frame with K-wires drilled through the frame and into the heel.
The long axis of the tibia is then determined using guide rods built into the leg holder frame. Sets of guide rods in the anterior-posterior and medial-lateral planes are manipulated using fluoroscopy to align the long axis of the tibia with the leg holder.
After an incision is made in the bottom of the heel, a drill guide built into the leg holder frame is placed against the undersurface of the calcaneus. The frame and alignment rods are designed to position the drill guide along the central axis of the tibia. A drill bit is used to make a channel up through the calcaneus and talus, and into the distal tibia. This drill bit follows the central axis of the tibia, established by the alignment of the drill guide built into the leg holder frame.
The size of the implants to be used is based on the size of the ankle bones seen on the fluoroscopy views. There are 5 sizes of implant sets and a saw guide that corresponds to each size. Each set of implants comprises a tibial implant, a talar implant and a polyethylene spacer.
The tibial implant is constructed from different components. The inferior part is a tibial tray. This has a set size and morphology specified by the size of the implant set chosen. Superior to this is a base, which also has a set size and morphology specified by the size of the implant set chosen. Superior to this are a variable number and size of stem components that are chosen by the surgeon during the procedure to give the best fit in the tibial intramedullary canal.
The talar component has a set size and morphology specified by the size of the implant set chosen. There is a stem that fits into the inferior portion of the talar implant and extends inferiorly either 10 mm or 14 mm at a defined angle. The choice of which stem length to use is made by the surgeon during the procedure.
Each implant set has a defined number of polyethylene spacers of varying height that fit into the tibial tray on the tibial implant. The height of the spacer to be used is chosen by the surgeon during the procedure, after the tibial and talar components have been fit into the bones.
Additionally, each implant set has a saw guide that corresponds to the size of the implants. Each saw guide has four slits built into it that allow passage of a saw, and define the orientation of the bone cuts. A slit for a superior cut is made in the distal tibia. A parallel inferior cut is made in the superior talus defined by an inferior slit. Oblique medial and lateral cuts are made in the distal tibia and onto the superior talus with the two other slits. When viewed from anterior the guide defines a trapezoidal set of cuts in the ankle.
The saw guide fits into the top of the leg holder frame and can be moved about above the anterior surface of the ankle joint, and will superimpose with the ankle bones when viewed with fluoroscopy. The ankle is still held in the position that gives the mortise view. Using fluoroscopy, the center of the saw guide is aligned with the drill bit in the central axis of the tibia. It is positioned to make parallel superior and inferior cuts that take a similar depth of bone from the dorsum of the talus and inferior surface of the tibia. The medial cut into the medial malleolus should be less than ⅓ of the width of that segment of bone. The lateral cut should just come against the medial surface of the lateral malleolus without cutting into it. When viewed in the medial-lateral plane with fluoroscopy, the cuts should be perpendicular to the long axis of the tibia and the bottom of the foot. The size of saw guide that meets these parameters is then chosen. This determines the size of the implant set to be used.
Once the chosen size saw guide is properly positioned over the anterior ankle, it is stabilized with K-wires drilled through it and into the tibia and talus bones. Then a saw is used to make the tibial and talar cuts through the slits in the guide. The guide is removed and the cut bone segments are also taken out. This leaves a trapezoidal space between the bones into which the implant set will fit.
The distal tibia is then further prepared by reaming along its central axis to accommodate the proper size of the tibial stem. The diameter of the channel to be reamed is determined by the size of the stem to be used. To ream the tibia, the proper diameter reamer bit is put into the ankle space through the anterior wound. A reamer driver is passed through the drill guide against the bottom of the calcaneus, and superiorly through the channel made by the drill bit into the ankle space to meet the reamer bit. After the driver engages the reamer bit, reaming along the central axis of the distal tibia is performed. The frame and guide ensure that the reaming is done with the proper alignment along the central axis.
The talus is finally reamed for the stem of the talar prosthetic component. The position for talar reaming is determined by a guide attached to the frame, and is based on the central axis of the tibia.
Although the above delineated technique is accurate, its complexity has disadvantages. For example, the frame that holds the leg must be constructed sterilely for each patient, a process that takes up valuable operating room time. Additionally, the process of determining the proper alignment of the ankle in the frame prior to cutting the bone is technically exacting, and also time-consuming. There is often a significant amount of fluoroscopic imaging required during the alignment process. Furthermore, all of the equipment used for this procedure must be processed for each separate use.
Another problem with this system, and with all of the existing systems for total ankle replacement, is difficulty in correcting angular deformities that are present. Most arthritis in the ankle is secondary to pre-existing trauma, and it is not uncommon for patients undergoing total ankle replacement to have malalignment at the ankle, or concurrent malalignment in the hindfoot. If an external frame is used to hold the leg, it maintains the relationship between the ankle bones, and the hindfoot, even if there is malalignment. Existing cutting guides used with these systems make both the tibial and talar bone cuts simultaneous with one saw guide. The cuts in the two bones are thus linked and are strictly dependent on the position of each bone relative to the other. Consequently, if an abnormal angular relationship exists between the two bones, it will be maintained after the bone cuts have been made.
Moreover, techniques have been developed to make custom bone cutting guides for knee joint replacement surgery. In those techniques, preoperative CAT scan or MRI scans are analyzed, and plastic models fabricated to fit against surfaces of the bones at the knee. Those plastic guides have appropriate slits placed in them that define the position that a saw blade can be inserted against the bone. The placement of those slits in the guides is made based on an analysis of the morphology of the bones and joint for the particular patient, combined with an understanding of the size and shape of the particular artificial joint prosthesis that will be used.
Accordingly, there is a need for a system for use in total ankle replacement surgery that overcomes the significant shortcomings of the known prior-art as delineated hereinabove.